The present invention relates to a preexposure device for printing forms or printing plates to be imagewise exposed. More particularly, the present invention relates to a preexposure device comprising a housing which is closed off at the bottom by a plate and in which a radiation source is disposed.
From DE-B 12 14 085 (U.S. Pat. No. 3,144,331) it is known to expose photopolymerizable recording materials which have been applied as a photosensitive layer to a printing plate support and whose sensitivity has fallen due to absorption of molecular oxygen, using 70 to 98% of the radiation dose of an actinic radiation which would be necessary with uniform incidence to initiate a photopolymerization in order to restore the sensitivity. The exposure is carried out through the printing plate support, which is, for example, transparent, an actinic radiation being used which has a wavelength such that only 10 to 70% of the radiation is absorbed by the photopolymerizable layer. In this process, exposure is in principle carried out first diffusely and then image-wise. The diffuse or preexposure is carried out with lower intensity, namely with 70 to 98% of the radiation intensity which is necessary to achieve the full exposure action. This preexposure is then followed by the imagewise exposure with full radiation intensity.
U.S. Pat. No. 4,298,803 discloses a process in which a photoresist layer is preexposed with an intensity which is less than the critical exposure intensity at which the photoresist can be substantially dissolved away at the exposed points. After this preexposure, the imagewise exposure of the photoresist layer is carried out. The sequence of the two exposures can be interchanged. In both cases, the photosensitivity of the photoresist is improved, which considerably shortens the processing time. In the device used for this process, both the imagewise exposure and the preexposure or afterexposure of the photoresist can be carried out by means of an electron beam, or of a UV or X-ray source.
U.S. Pat. No. 4,716,097 likewise discloses a process in which a photopolymeric layer containing a dyestuff is first diffusely and then imagewise exposed to light having a wavelength above 400 nm and an intensity of at least 1500 lumens/m.sup.2.
German Offenlegungsschrift DE-A 24 12 571 discloses a process for curing a light-curable polymer layer of a printing plate, in which exposure is first carried out diffusely for a short time and then imagewise until the polymer layer is virtually completely cured in the exposed areas. The diffuse exposure duration is not more than 90% of the time within which the complete curing of the polymer layer takes place with equal intensity of radiation both for the preexposure and for the imagewise exposure.